I honestly can never use vsync in any of my games. It leads to a slight mouse lag which completely screws me up. If you want to overclock cards in crossfire with MSI Afterburner you can, heres how:

Open up regedit
Go to: HKEY_LOCAL_MACHINE>>SYSTEM>>ControlSet001>>Control >>Class>000
When you are in the 000 folder select EnableUlps. By default there will be a 1, change it to a 0.
Restart your computer.
You can now overclock your cards using AB without any crashes. Once I get my second card on water I'll let you know how it handles extra voltage going to both.